Non-aqueous electrolyte batteries which have recently been utilized as main electric sources of mobile communications equipment and portable electronic equipment have the features of high electromotive force and high energy density. As positive electrode materials used in these non-aqueous electrolyte batteries, there are known composite oxides of lithium and transition metals having electrons in the 3 d orbit, such as lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), and the like. Especially, lithium manganese composite oxides have the advantages that they are less in environmental pollution and cheaper than other oxides, and, from this viewpoint, research and development of the lithium manganese composite oxides are now being conducted intensively.
According to the research and development, in addition to the improvement of battery capacity and safety, the following problems are investigated.
Since there is a great need for non-aqueous electrolyte batteries as electric sources, particularly, of portable mobile terminals, various environments of use are supposed. Therefore, environmental tests are indispensable in development of batteries. For example, such tests are necessary which are conducted on the supposition that they are used in an atmosphere of high temperature and high humidity, in an environment of low temperature, and the like.
Especially, when non-aqueous electrolyte batteries are used or stored in an environment of high temperatures such as the inside of cars in summer, battery performances such as battery capacity and cycle characteristics are sometimes damaged.
Furthermore, the deterioration of battery performances of non-aqueous electrolyte batteries stored in an environment of high temperatures causes further serious problems in non-aqueous electrolyte batteries which use lithium manganese composite oxides as positive electrode materials.
The direct cause for the above problems is that the manganese ion dissolves out of the lithium manganese composite oxide which is an active material of positive electrode. From this viewpoint, investigation has been made on materials which inhibit the dissolution of the manganese ion from the active material of positive electrode. For example, JP-A-9-73902 discloses a technique of controlling the amount of sodium contained in lithium manganese composite oxide of the positive electrode to 0.1–0.8% by weight to remove impurities contained in the material, thereby inhibiting dissolution of manganese ion. Moreover, JP-A-9-82360 discloses a technique of inhibiting dissolution of manganese ion by covering the surface of lithium manganese composite oxide which is a positive electrode material with a lithium ion conductive solid electrolyte.
These techniques are based on the standpoint of reducing the reactivity of the electrolyte with the active material of positive electrode and are effective for inhibiting deterioration of battery characteristics in the case of using or storing the batteries in an environment of high temperatures.